From now on, it shouldn't complain about a missing module,
if in fact it is already fetched

If a module has "fetchto" variable defined, then all
it's modules are fetched to this destination. If the
variable is indefined, then it is taken the parent.

Now modules to be fetched are recognized properly

Add an option to the ModulePool for building list of all files
that are engaged in a project. This includes files that
are not listed in the manifests but are included in the
verilog sources